This application claims priority of No. 106111641 filed in Taiwan R.O.C. on Apr. 7, 2017 under 35 USC 119, the entire contents of which are hereby incorporated by reference.
Field of the Invention
The invention relates to a power line communication device and method, and more particularly, to a power line communication device and method capable of selecting channels and modulation mechanisms.
Description of the Related Art
Power line communications (PLC) include systems for transmitting data over the same medium (i.e., a wire or conductor) that is also used to transmit electric power to residences, buildings, and other premises. U.S. Pat. No. 9,172,431 discloses a power distribution system for power line communication. FIG. 1 shows a schematic diagram of a conventional electric power distribution system. As shown in FIG. 1, Medium voltage (MV) power lines 103 from substation 101 typically carry voltage in the tens of kilovolts range. Meters 106a-n are typically mounted on the outside of any type of residences 102a-n for receiving or consuming electricity. Transformer 104 steps the MV power down to low voltage (LV) power on LV lines 105, carrying voltage in the range of 100-240 VAC. Transformer 104 is typically designed to operate at very low frequencies in the range of 50-60 Hz. Transformer 104 does not typically allow high frequencies, such as signals greater than 100 KHz, to pass between LV lines 105 and MV lines 103. LV lines 105 feed power to customers via meters 106a-n. Panel 107 provides an interface between meter 106n and electrical wires 108 within residence 102n. Electrical wires 108 deliver power to outlets 110, switches 111 and other electric devices within residence 102n. 
PLC gateways (or modems) 112a-n at the residences 102a-n may use MV/LV lines 103/105 to carry data signals to and from PLC data concentrator 114 without requiring additional wiring. Concentrator 114 may be coupled to either MV line 103 or LV line 105. Modems or gateways 112a-n may support applications such as high-speed broadband Internet links, narrowband control applications, low bandwidth data collection applications, or the like. PLC gateways 112a-n may enable AC or DC charging of electric vehicles. An example of an AC or DC charger is illustrated as PLC device 113. The above-mentioned power line communication networks may provide street lighting control and remote power meter data collection.
Data concentrators 114 may be coupled to control center 130 (e.g., a utility company) via network 120. Network 120 may include, for example, the Internet, a cellular network, a WiFi network, a WiMax network, or the like. As such, control center 130 may be configured to collect power consumption and other types of relevant information from gateway(s) 112 and/or device(s) 113 through concentrator(s) 114.
At present, the power line communication system has been widely used in the Smart Grid. The conventional power line communication protocol can be roughly divided into two types. The first one adopts a narrowband OFDM as a modulation mechanism and the second one adopts a wideband OFDM. The power line communication protocol of a narrowband OFDM includes G3, PRIME, IEEE1901.2 and Ghnem, which use the bandwidth from 200˜600 KHz. Currently, the protocol of a broadband OFDM includes only IEEE1901.1 which is still under development and uses signal bandwidth up to 12.5 MHz, being 20 times the narrowband OFDM.
There is no channel switching mechanism in the narrowband OFDM protocol, and IEEE1901.1, a wideband OFDM protocol, has established a channel switching mechanism which is mainly controlled by a concentrator 114 through beacons. OFDM modulation takes full advantage of high-speed bandwidth transmission and has a very good tolerance for multi-path channel. However, OFDM is still unable to achieve the same stable transmission as Spread Spectrum at very low signal to noise ratios.
The channel switching mechanism is required since each sub-channel has different channel quality. If the best sub-channel transmission is chosen, the communication may be more rapid and stable. According to G3, PRIME, IEEE1901.2, Ghnem and other narrowband OFDM communication protocol, there is no channel switching mechanism since the channel is too narrow. The channel switching mechanism stipulated by IEEE1901.1 in v0.1 version is controlled by a concentrator 114 through beacons. This approach has the following disadvantages.                1. In the 50 Hz power line system, the period of the beacons of the concentrator 114 is 40 ms, so that the switching can only be performed every 40 ms.        2. Because the channel switching is controlled centrally by the concentrator 114, each meter in the power line network must use the same sub-channel at the same time point. However, the best sub-channels for different meters are actually different. As a result, this switching method cannot be optimized for all meters.        3. When there are many meters in the network, there will be some meters located far away from the concentrator 114. The beacons sent by the concentrator 114 may not be received by these meters, which become hidden nodes. This can cause the meters to go offline and must be reconnected with the concentrator 114, so that the overall performance of the entire network is reduced.        4. The timing of channel switching for all meters must be universal. If the switching time have differences or error among meters, there will be packet loss. When the meters are made by different manufacturers, the differences or error of the switching time will be more serious.        
OFDM modulation takes full advantage of high-speed data transmission by efficiently using the bandwidth and has a very good tolerance for multi-path channels. However, OFDM is still unable to achieve the same stable transmission as Spread Spectrum at very low signal to noise ratios. The smart grid utilizing OFDM modulation can only cover narrower range. In a large community having thousands of households, the meter-reading may not be successful for the meters at the outskirt of the grid.
FIG. 2 shows a schematic diagram of a conventional frame format. As shown in FIG. 2, the format of the conventional frame 120 includes a preamble 121, a frame control header 122 and a payload 123. The preamble 121 includes a plurality of synchronization symbols P1-P6 and M1, which are used as signals for synchronization. In addition, there are only synchronization symbols in the preamble of the protocols such as G3, PRIME, IEEE1901.2, Ghnem and IEEE1901.1.
For the purpose of channel selection, U.S. Pat. No. 9,172,431 proposes a technique of using a beacon to achieve channel selection in power line communications. The method includes steps of defining a plurality of frames, each of the frames having a plurality of time slots, assembling a pair of beacons within each time slot in each frame, and then selecting the channels by scanning the frames and the beacons. However, the technique of the above patent still has the aforementioned problems. Therefore, a new solution is needed.